RAND_add, RAND_seed, RAND_status, RAND_event, RAND_screen
- Add entropy to the PRNG
const void *buf, int num ); void RAND_add(
const void *buf, int num, double entropy ); int
void ); int RAND_event(
UINT iMsg, WPARAM wParam, LPARAM lParam ); void
The RAND_add() function mixes the num bytes at buf into
the PRNG state. Thus, if the data at buf are unpredictable
to an adversary, this increases the uncertainty about the
state and makes the PRNG output less predictable. Suitable
input comes from user interaction (random key presses,
mouse movements) and certain hardware events. The entropy
argument is (the lower bound of) an estimate of how much
randomness is contained in buf, measured in bytes. Details
about sources of randomness and how to estimate their
entropy can be found in the literature, e.g. RFC 1750.
The RAND_add() function may be called with sensitive data
such as user entered passwords. The seed values cannot be
recovered from the PRNG output.
OpenSSL makes sure that the PRNG state is unique for each
thread. On systems that provide /dev/urandom, the randomness
device is used to seed the PRNG transparently. However,
on all other systems, the application is responsible
for seeding the PRNG by calling the RAND_add(), RAND_egd()
or RAND_load_file() functions.
The RAND_seed() function is equivalent to the RAND_add()
function when num == entropy.
The RAND_event() function collects the entropy from Windows
events such as mouse movements and other user interaction.
It should be called with the iMsg, wParam and
lParam arguments of all messages sent to the window procedure.
It will estimate the entropy contained in the event
message (if any), and add it to the PRNG. The program can
then process the messages as usual.
The RAND_screen() function is available for the convenience
of Windows programmers. It adds the current contents
of the screen to the PRNG. For applications that can
catch Windows events, seeding the PRNG by calling
RAND_event() is a significantly better source of randomness.
It should be noted that both methods cannot be used
on servers that run without user interaction.
The RAND_status() and RAND_event() functions return 1 if
the PRNG has been seeded with enough data, 0 otherwise.
The other functions do not return values.
The RAND_seed() and RAND_screen() functions are available
in all versions of SSLeay and OpenSSL. The RAND_add() and
RAND_status() functions have been added in OpenSSL 0.9.5,
and RAND_event() in OpenSSL 0.9.5a.
Functions: rand_ssl(3), RAND_egd(3), RAND_load_file(3),
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